A conventional loudspeaker (hereinafter “speaker”) technology is described referring to FIG. 7, a cross sectional view of a speaker. FIG. 8 shows a cross sectional view of another conventional speaker. A conventional speaker of FIG. 7 comprises a frame 1, a yoke 2, a magnet 3, a top plate 4, a diaphragm 9, an edge 10, a damper 11, a voice coil 12, a dust cap 14 and a magnetic gap 15. The main objective of the edge 10 is to hold the diaphragm 9 at a center portion and to close a cavity behind the diaphragm 9, while that of damper 11 is to keep the diaphragm 9 at the center and to control the lowest resonance frequency (hereinafter referred to as “f0”) by making use of the damper's flexibility.
The higher the flexibility of damper 11, the higher the flexibility of the speaker as a whole, and the lower f0 becomes. Since the lowest frequency a speaker can reproduce is substantially determined by f0, specifying a flexibility for damper 11 is one of the key factors in designing the sound reproduction characteristics in a low frequency range.
As a result of expanded reproduction frequency range brought about as a result of recent progress in the digital technologies, the speakers are required to be able to reproduce still lower frequency sounds. Some of the speakers are requested to provide a low f0 that did not exist before.
If in a conventional speaker the flexibility of the damper 11 is increased with the aim of simply lowering f0, the capability of the damper to keep the diaphragm at the center becomes weak and a supporting state of the vibration system becomes unstable. Under such a state, the vibration system readily causes a rolling motion, which makes the voice coil 12 physically contact with yoke 2 or top plate 4 in the magnetic gap and generate abnormal noise or deteriorated sound. In the worst case, it leads to a breakdown of the voice coil 12 and vibration system.
A speaker proposed to solve the above-described problems is described referring to FIG. 8. Only the differences from the one illustrated in FIG. 7 are described. The improved speaker as shown in FIG. 8 further comprises a bearing 5, a shaft 8 and a center cap 13, besides the conventional constituent elements. The shaft 8 is fixed to the center cap 13 at the center, the center cap being fixed on the upper end of a voice coil bobbin. The shaft 8 is supported by the bearing 5 fixed in a through hole formed in a magnetic circuit. Namely, the vibration system is supported by the shaft 8, instead of the damper 11 used in the speaker of FIG. 7. An elimination of damper 11 results in an advantage that is equivalent to an extremely flexible damper 11, while the centering of the vibration system is well maintained.
In the conventional speaker as shown in FIG. 8, however, there exists friction between the shaft 8 and the bearing 5, and resonance occurs in a gap between the shaft 8 and the bearing 5. The resonance is transferred via shaft 8 to the diaphragm 9, and magnified to cause an abnormal sound. In order to prevent such drawbacks, a countermeasure, for example, inserting a sound absorbing material between the shaft 8 and the diaphragm 9, needs to be provided. However, the countermeasure impairs rigid supporting of the shaft 8; more importantly, it cannot prevent generation of abnormal sounds completely.
The present invention addresses the drawbacks of the conventional speakers, and aims to provide a speaker in which an abnormal sound is suppressed and the vibration system can move at great amplitudes.